Pulsar observed 'powering up'

An international team of astronomers has observed for the first time the full process of material spiralling into a distant neutron star, triggering an X-ray outburst thousands of times brighter than our Sun.

The scientists observed an ‘accreting’ neutron star as it entered an outburst phase, in a collaborative effort led by Monash University PhD candidate Adelle Goodwin and involving five groups of researchers, seven telescopes (five on the ground, two in space) and 15 collaborators. This is the first time such an event has been observed in this detail — in multiple frequencies, including high-sensitivity measurements in both optical and X-ray.

In an accreting neutron star system, a pulsar (a dense remnant of an old star) strips material away from a nearby star, forming an accretion disk of material spiralling in towards the pulsar, where it releases extraordinary amounts of energy — about the total energy output of the Sun in 10 years — over the period of a few short weeks.

The physics behind this ‘switching on’ process has eluded physicists for decades, partly because there are very few comprehensive observations of the phenomenon. The researchers caught one of these accreting neutron star systems in the act of entering outburst, revealing that it took 12 days for material to swirl inwards and collide with the neutron star — substantially longer than the 2–3 days most theories suggest.

“These observations allow us to study the structure of the accretion disk and determine how quickly and easily material can move inwards to the neutron star,” Goodwin said.

“Using multiple telescopes that are sensitive to light in different energies we were able to trace that the initial activity happened near the companion star, in the outer edges of the accretion disk, and it took 12 days for the disk to be brought into the hot state and for material to spiral inward to the neutron star, and X-rays to be produced.”

Accretion disks are usually made of hydrogen, but this particular object has a disk that is made up of 50% helium — more helium than most disks. The scientists think that this excess helium may be slowing down the heating of the disk because helium ‘burns’ at a higher temperature, causing the powering up to take 12 days.

“This work enables us to shed some light on the physics of accreting neutron star systems and to understand how these explosive outbursts are triggered in the first place, which has puzzled astronomers for a long time,” said study co-author Dr David Russell, from New York University Abu Dhabi.

The pulsar observed was SAX J1808.4−3658, which rotates at a rapid 400 times per second and is located 11,000 light-years away in the constellation Sagittarius. The research is being featured at this week’s American Astronomical Society meeting before it is published in the journal Monthly Notices of the Royal Astronomical Society.

Image caption: An accreting pulsar. Image credit: NASA/JPL-Caltech.

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